This invention relates to overbased lubricating oil additives. More particularly, it relates to an improved method of preparing overbased magnesium sulfonates with alkali values over 400 and low viscosities, starting from commercially available grades of magnesium oxide.
Although overbased additives have been used in lubricant formulations for many years, their structure is still a matter of some controversy and their preparation is a complex and highly unpredictable art. An overbased additive consists essentially of a dispersing agent dissolved in a diluent oil, in combination with substantial quantities of a basic compound, usually inorganic, in the form of a submicronic colloidal dispersion. The preparation of such dispersions is customarily referred to as "overbasing". Presumably the dispersing agent exists in the form of micelles, in which the colloidal particles of basic compound are incorporated. Numerous combinations of oil soluble dispersing agent and colloidally dispersed base have been prepared, but the most widely used are the overbased sulfonates. These comprise the calcium, barium, and magnesium salts of oil soluble sulfonic acids in combination with colloidally dispersed calcium, barium, and magnesium carbonates. When employed in a lubricant such as an automobile crankcase oil, the carbonate serves to neutralize potentially corrosive acidic contaminants formed either by oxidation of the oil or partial combustion of the fuel, while the oil soluble sulfonate, in addition to dispersing the carbonate, also functions as a detergent to maintain engine cleanliness, and moreover imparts some degree of rust protection to susceptible metal parts.
The requirements for a commercially acceptable overbased sulfonate are formidable. In general, it must have an alkali value (AV) of at least 250 milligrams of KOH per gram equivalent--that is, each gram of overbased sulfonate must be capable of neutralizing as much acid as 250 milligrams of potassium hydroxide. Simple chemical calculation will show that a "250 AV" overbased magnesium sulfonate must contain 19% by weight dispersed magnesium carbonate. Likewise, a "400 AV" overbased magnesium sulfonate must contain 30% magnesium carbonate. This carbonate must be in such a fine state of subdivision that it will not separate from the additive on standing and cannot be removed from the lubricant in which the additive is employed by simple in-line filtering devices such as the oil filter in an automobile. An acceptable overbased sulfonate will be clear and transparent to the naked eye, even though it contains 20-30% of a highly insoluble metal salt. Any haze or cloudiness signals the presence of large particles, which may settle out causing loss in neutralizing power and possible abrasion of metal surfaces. Furthermore, an acceptable overbased sulfonate must have a viscosity sufficiently low that it can be transferred and blended in a plant without trouble. This last requirement is not always simple to meet. As the concentration of dispersed carbonate is increased, there is a marked tendency for the additive to thicken, even to the point of gelation and, although a successfully fine colloidal dispersion may have been achieved, the product may be hopelessly intractable. Thus, although numerous methods have been proposed for the preparation of overbased sulfonates, relatively few are capable of producing an additive of high alkali value (250 or above) that is commercially acceptable. And, although calcium, barium, and magnesium are all alkaline earth metals, many of their compounds differ considerably in solubility and reactivity with the result that a process that will yield a useful overbased sulfonate of one of these metals will not necessarily be commercially successful when applied to another.
This fact is particularly apparent when dealing with magnesium. Calcium and barium overbased sulfonates can both be prepared from the corresponding oxides. The general method comprises forming a mixture of the oxide, water and/or alcohol, an alkylbenzene sulfonate salt in a diluent oil, and a petroleum solvent, adding thereto carbon dioxide until the oxide is converted to the carbonate, and then removing water, alcohol, solvent, and undispersed particles to obtain the overbased sulfonate product in the form of a colloidal dispersion in the diluent oil. Of course, the process is not as simple as this brief description may suggest. The objective is not merely to prepare calcium or barium carbonate by the reaction of the oxide with CO.sub.2, but rather to prepare it in the form of a highly concentrated stable submicronic colloidal dispersion, transparent to the naked eye. Careful attention to temperature, solvent, type of oxide, type of dispersing agent, etc. is critical. Isolation of the product, if not carried out with scrupulous care, may cause coagulation of the colloidal carbonate particles or formation of an intractable gel. However, these problems have largely been overcome insofar as calcium and barium are concerned. Overbased calcium sulfonates of 250 AV or higher are routinely manufactured by various versions of the above oxide process.
Unfortunately, the oxide method has been considerably less successful when applied to magnesium. Hazy products with low AVs are often obtained, and much of the oxide ends up as undispersed solid which is difficult to filter. There are undoubtedly many reasons, but one of the biggest factors is the enormous variation in the activity of commercially available grades of magnesium oxide. Magnesium oxide (magnesia) is normally manufactured by high temperature decomposition (calcination) of various oreas--magnesite (magnesium carbonate), dolomite (a mixed carbonate of calcium and magnesium), or brucite (magnesium hydroxide). It has also been manufactured from magnesium chloride and magnesium sulfate. If the calcination is carried out at relatively high temperatures (e.g. 1600.degree. C.), the resulting oxide is dense, refractory, and fairly inert chemically, and is customarily referred to as "dead burned" or "heavy" magnesia. Magnesium oxides prepared by calcination at lower temperatures (e.g. 600.degree.-900.degree. C.) are less dense and more reactive, and are usually referred to as "light", "active", or "caustic burned" magnesias. It is these latter grades which have normally been used in the attempted preparation of overbased magnesium sulfonates. However, there is considerable variation in the reactivity of different grades of "active" magnesia, depending on the exact calcination temperature employed, the composition and the quality of the ores calcined, etc. The total surface area, the microscopic pore diameter, and the crystal form may differ dramatically between two different "active" magnesias. Even the same grade of magnesium oxide from the same manufacturer may show significant variations in quality from one year to the next. Thus an overbasing procedure which works reasonably well with one form of "active" magnesium oxide may fail when a different oxide, or even a different lot of the "same" oxide, is used. Attempts have been made in the prior art to overcome this problem by the addition to the overbasing reaction mixture of "promoters" such as alcohols, ammonia, amines, and salts thereof, phenols, and naphthenic acids, in order to increase the reactivity of the magnesium oxide. However, these have not solved the basic problem--namely, that the manufacturer of overbased sulfonates usually has little or no control over the quality of the magnesium oxide on which the success of his process depends. Thus most commercially available magnesium overbased sulfonates have heretofore been manufactured, not from magnesium oxides, but from the more expensive magnesium metal. The metal is dissolved in an alcohol and simultaneously or subsequently contacted with carbon dioxide to form a soluble alkoxymagnesium carbonate complex, which is added to a magnesium alkylbenzene sulfonate in a petroleum diluent and hydrolyzed to the desired magnesium carbonate dispersion--see, for example, Hunt, U.S. Pat. No. 3,150,089 and Dickey, U.S. Pat. No. 3,761,411.